Not applicable.
Not applicable.
This invention generally relates to the field of roadway paving and finishing devices. More specifically, this invention relates to a screed assembly having improved ability to control the magnitude of a change in the angle of attack of the screed plate in response to a change in the angle of the tow arms and to control the magnitude of a change in the angle of the tow arms in response to a change in the angle of attack.
Conventional paving and/or finishing assemblies typically consist of two basic units, a tractor or other vehicle capable of distributing paving material (e.g., asphalt or other bituminous materials) and a screed device. The screed device includes a screed platform, a screed plate forming the bottom surface of the platform and a means for towing the platform behind the tractor. During the process of laying down paving material, the screed platform typically is towed behind the tractor by a single pair of tow arms, one arm coupled with one side of the tractor and screed platform and the other arm coupled with the opposing side of the tractor and screed platform. In operation, the tractor tows the screed device in a forward direction with the screed plate contacting the surface to be paved or finished to level and/or smooth the surface.
As the screed device is towed behind the tractor, paving material is distributed under the screed platform. Consequently, the vertical force imparted to the screed plate is increased and the platform is vertically displaced from the existing surface by the applied paving material. The magnitude of this vertical displacement is dictated, in part, by the characteristics of the material and the displacement, in turn, determines the depth or thickness of the layer of material being laid down, i.e., the mat. Irregularities in the surface being paved or in the material properties (e.g., the viscosity or density) of the material being laid down also can affect the vertical displacement of the screed platform. Controlling the magnitude of this vertical displacement is crucial in producing high-quality mats which are of uniform thickness and texture.
The angle between the bottom surface of the screed platform (i.e., the screed plate) and the surface over which paving is being performed commonly is referred to as the xe2x80x9cangle of attackxe2x80x9d. This angle is a primary factor in determining the amount of material that will pass under the screed platform over a given distance and is elemental in controlling mat depth. The angle of attack will always seek an equilibrium, i.e., a substantially constant magnitude wherein the screed plate is moving in a plane essentially parallel to the grade surface. With the standard screed assembly, the magnitude of any change in the angle of attack is equal to the magnitude of any change in the tow angle. The tow angle is the angle, measured at the tow point (i.e., the point at which the tow arms are connected to the tractor), of the tow arms with respect to horizontal. The magnitude of a change in the two angles is the same, but the direction of the change is opposite, i.e., if one increases the other decreases. This result is determined by the linkage used to couple the towing vehicle and the screed platform, namely, a single pair of tow arms, one coupled with each side of the tractor and the screed platform. This linkage causes the entire screed device to act as a single lever, pivoting at the tow point.
As a standard screed platform is pulled behind the towing vehicle, the angle of attack may change intentionally or unintentionally. For instance, if the towing vehicle proceeds over an uneven surface, the angle of attack will change due to the vertical displacement of the vehicle and such changes are undesirable and unintentional. The angle of attack may also be adjusted intentionally by use of a depth crank. Such changes in the angle of attack, whether intentional or unintentional, result in mats that are either thicker or thinner, depending upon the direction of the change. For example, if the density of the paving material decreases, the vertical force imparted to the screed plate also decreases causing the screed plate to drop until the screed platform reaches an equilibrium, i.e., until the screed plate is again moving in a plane essentially parallel to the grade surface. This drop simultaneously increases the angle of attack and decreases the tow angle by the same magnitude. This permits less material to pass beneath the platform and causes the mat depth to decrease. Alternatively, if the density of the paving material increases, the vertical force imparted to the screed plate also increases causing the screed plate to rise until the screed platform reaches an equilibrium. This rise decreases the angle of attack and increases the tow angle by the same magnitude. This permits more material to pass beneath the platform and causes the mat depth to increase.
Paving over a flat, level surface with all variables held constant, a conventional screed assembly will produce a mat of constant profile. However, unevenness of the surface being paved can change the mat profile. When the plane in which the tractor is traveling changes, e.g., due to irregularities in the surface being paved, the tow angle changes accordingly. As a result, the angle of attack changes as does the thickness of the mat. Accordingly, due to irregularities in the surface being paved, evenness in the final thickness of the mat may vary. Such is not a desirable result as a smooth, even mat of uniform thickness is desired. In such circumstances, it would be desirable to have a screed assembly in which a change in the angle of attack was lesser in magnitude than the corresponding change in the tow angle rather than equal.
Circumstances also exist when it is desirable for the angle of attack to change more rapidly and/or more dramatically than the tow angle. For example, a change in the properties of the paving material, e.g., the viscosity or aggregate properties, may cause a change in the angle of attack and thus the thickness of the mat which is not desired. In such circumstances, it would be desirable to have a screed assembly in which a change in the angle of attack was greater in magnitude than the corresponding change in the tow angle rather than equal so as to compensate for the change in physical properties of the applied material.
Accordingly, there is a continuing need in the paving and finishing industry for a screed device which has improved ability to control the magnitude of the change in the angle of attack of the screed plate in response to a change in the tow angle and to control the magnitude of the change in the tow angle in response to a change in the angle of attack. Additionally, there remains a need for a screed device which may be coupled with the towing vehicle in a manner other than that typically used in the industry providing for improved sensitivity and response in the angle of attack.
Accordingly, in one of its aspects, the present invention provides a screed assembly having improved ability to control the magnitude of a change in the angle of attack of the screed plate in response to a change in the tow angle such that a change in the angle of attack is not constrained to be equal in magnitude to a change in the tow angle.
In another of its aspects, the present invention provides a screed assembly having improved ability to control the magnitude of a change in the tow angle in response to a change in the angle of attack such that the change in the tow angle is not constrained to be equal in magnitude to a change in the angle of attack.
In a further aspect, the present invention provides a screed assembly having dual sets of tow arms, one set coupled on each side of a prime mover, wherein each set includes at least two tow arms.
In another of its aspects, the invention provides a method of maintaining a more consistent mat depth despite unevenness in the surface to which material is being applied or inconsistencies in the properties of the applied material which comprises pivotally coupling dual sets of tow arms to a paving vehicle, one set on each side of the vehicle, and towing the screed plate over a surface to be paved.
In an additional aspect, the invention provides a method whereby the magnitude of a change in the angle of attack due to inconsistencies in the properties of the applied material is greater than the magnitude of a corresponding change in the angle of the tow arms resulting in a more consistent mat depth.
In a further aspect, the invention provides a method whereby the magnitude of a change in the angle of attack due to unevenness in the surface to which material is being applied which is lesser than the magnitude of a corresponding change in the angle of the tow arms resulting in a more consistent mat depth.
According to the present invention, the foregoing and other aspects are achieved by a screed device for use with a prime mover adapted to be moved over a surface to distribute a material on the surface. The device of the present invention comprises a screed platform, the platform having a screed plate which forms a bottom face thereof, and first and second sets of tow arms operably coupled between the prime mover and the screed platform. Each set of tow arms includes at least two arms and the tow arms of each set are pivotally coupled with the prime mover and the screed platform. In one embodiment, each set of tow arms includes two arms and the pivotal coupling of one arm is vertically spaced from the pivotal coupling of the other arm of the same set.
Aspects of the present invention are further achieved by a roadway paving assembly which comprises a vehicle, a screed platform and first and second sets of tow arms operably coupled between the vehicle and the screed platform. The vehicle of the paving assembly is adapted to be moved over a surface to distribute a material on the surface. Each set of tow arms includes at least two arms and the individual arms of each set are pivotally coupled with the vehicle and the platform. In one embodiment, each set of tow arms includes two arms and the pivotal coupling of one arm is vertically spaced from the pivotal coupling of the other arm of the same set.
Still further, aspects of the present invention are achieved by a method of maintaining a substantially consistent depth of material applied to a surface utilizing a screed platform which is towed by a prime mover, the platform having a screed plate which forms a bottom face thereof. The method of the present invention comprises providing first and second sets of tow arms adapted to extend between the prime mover and the screed platform, pivotally coupling each set of tow arms with the prime mover and the screed platform, and towing the screed platform behind the prime mover over the surface. Each set of tow arms includes at least two tow arms. In one embodiment, each set of tow arms includes first and second arms, the pivotal coupling of the first arm being vertically spaced from the pivotal coupling of the second arm of the same set.
Additional aspects of the invention, together with the advantages and novel features appurtenant thereto, will be set forth in part in the description which follows, and in part will become apparent to those skilled in the art upon examination of the following, or may be learned from the practice of the invention. The aspects and advantages of the invention may be realized and attained by means, instrumentalities and combinations particularly pointed out in the appended claims.